This invention is directed to small size pigments of less than about 35 nm in average diameter which are formed of an cationic dye coating adsorbed on negatively charged silica particles. The color pigments formed in this invention are useful in color films as well as in various applications such as color filters for liquid crystal displays and in various other applications such as in ink jet color printing for color filter applications.
The interactions between dyes and solid surfaces may involve covalent bond formation or physical forces (electrostatic and/or van der Waals). For example, reactive dyes were grafted to the surface of derivatized silicas to prepare water-dispersible pigments for ink-jets (Winnik et al., Dyes and Pigments, 14, 101 (1990)). A number of studies (See: McKay et al., J. Appl. Plym. Sci., 27, 3043 (1982); Fleming, H. L., "Fundamentals of Adsorption", pp. 221, Athanasios I. Liapis, Ed., Engineering Foundation, New York, 1987; Jain et al., Colloid Surf., 29, 373 (1988); Mishara et al., J. Colloid Interface Sci., 129, 41 (1984); Charreyre et al., J. Colloid Interface Sci., 170, 374 (1995); Jan et al., Colloid Surf., 92, 1 (1994)) dealt with the adsorption of dyes on charged solids, e.g. McKay et al., supra, investigated the adsorption of dyestuffs on chitin. Because of the porosity of the latter, it was difficult to establish the adsorption mechanism; over a limited dye concentration range, both Langmuir and Freundlich isotherms could fit the data. When alumina was used as adsorbent (See: Fleming, H. L., "Fundamentals of Adsorption", pp. 221, Athanasios I. Liapis, Ed., Engineering Foundation, New York, 1987; Jain et al., Colloid Surf., 29, 373 (1988); Mishara et al., J. Colloid Interface Sci., 129, 41 (1984)) the uptake of dyes was strongly dependent on the particle morphology, method of preparation, pretreatment of the solids and the equilibrium pH conditions. As a result of these issues, alumina particles are not considered practical for this application.
This invention discloses the formation of colored pigments by interacting several cationic dyes with negatively charged silica particles where the counter ion is ammonium or sodium. The advantage of this adsorbent is in its small size (&lt;30 nm) and the negative charge of the surface. Previous studies (See: Giesche et al., Dyes & Pigments, 17, 323 (1991); Hsu et al., Dyes & Pigments, 19, 179 (1992); Carotenuto et al., Ind. Eng. Chem. Res. 35, 2929 (1996); Tentorio et al., J. Colloid Interface Sci., 77, 418 (1980)) showed that adsorbing dyes on solid surfaces or incorporating them into inorganic particles can yield reproducible pigments of superior optical and mechanical properties.
Reference should also be had to the following U.S. Pat. Nos: 5,344,489, 5,318,797, 5,318,628, 5,248,556, 5,015,452, 4,755,373, 4,675,251 and 4,241,042 relating to pigments for further background information. Also background information relating to this field is to be found in WO 92/21726 published Dec. 12, 1992 and WO 94/21733 published Sept. 29, 1994.
This invention also is directed to color filters for a color liquid crystal display (LCD) and more particularly to imaging color filter resists such as negative tone color filter photoresists for both primary additive and secondary subtractive colors also utilizing nanosized colored pigments. It is also disclosed in this invention the use of top imaging positive or negative-tone photoresists prepared with nanosized pigments, through which color pattern is delineated via aqueous alkaline development or solvent removal.
It is essential for color filter application that the pigment particles yield transparent filters of high contrast. In order to achieve these properties, it is necessary for pigment particles to be small, uniform in size, and fully dispersed in the polymer, in order to minimize light scattering effects, which would adversely affect the transparency. Furthermore, for the same reason above, refractive index of the core material was found to be necessary for the subject application. The small and uniform size of pigments produced herein are found to meet these requirements. The silica core size in the range of less than 35 nm colored with dyes will yield completely transparent filters in said polymers. The low refractive index contributes to the transparency of color filters and will provide an unexpectedly high contrast.
In contrast, the pigments described in U.S. Pat. Nos. 5,318,628 and 5,344,489 are considerably larger particles, and as such, are not suitable for color filters or colored ink jets for making colored filters.
In addition, particles of higher refractive index in these patents would also not be suitable for colored filters because of significant scattering effects which reduces transparency.
For background description of a color filter for an LCD and the method of producing the same reference should be had to U.S. Pat. No. 5,278,009.